1. Field of the Invention
The present invention relates to a gamma corrector, and more particularly to a gamma corrector for operating data of a lookup table on input image data of two or more component colors to thereby obtain gamma-corrected image data for each component color.
2. Description of the Background Art
A liquid crystal display device, as shown in FIG. 1, generally includes a backlight, two sheets of glass, two polarizer plates formed on the outer surfaces of the two glass sheets, two electrodes coated on the inner surfaces of the two glass sheets, a color filter interposed between the electrode coating and the front glass sheet, and a layer of liquid crystal molecules sandwiched between the two electrode coatings. With light illuminated from the backlight toward the front glass sheet, a voltage is applied between the front and back electrode coatings to alter the orderly arrangement of the molecules of the liquid crystal layer. This causes the transmissivity of the polarized light to vary at each pixel, whereby an image is displayed.
However, it is known that the transmissivity of light depends upon the properties of a liquid crystal and is not proportional to a potential difference applied between two electrode coatings, that is, an input voltage as shown in FIGS. 2 and 3A. For this reason, display units are required to correct a voltage-transmissivity characteristic so that it becomes a proportional characteristic easy to control. This correction will be hereinafter referred to as gamma correction. As shown in FIG. 3B, after gamma-corrected, an input voltage is applied between the electrodes. As a result, the transmissivity of light exhibits a characteristic proportional to the input voltage, as shown in FIG. 3C.
Gamma correction, as shown in FIG. 4, has heretofore been performed with a source driver used in a liquid crystal display controller. In the source driver, as shown in FIG. 4, a logic value from a timing controller controlling the liquid crystal display controller is received, this logic value is converted into an analog voltage with a digital-to-analog (D/A) converter incorporated in the source driver, and the converted voltage is applied between liquid-crystal control electrodes employed in the liquid crystal display controller. The D/A controller has a function to output an analog voltage according to characteristics of a liquid crystal and uses this function to perform a gamma correction.
However, since liquid crystal displays differ from one another in characteristics of a liquid crystal, this method has to design a gamma correction circuit specifically for an individual liquid crystal display. An example of a method for solving this problem is to provide, as shown in FIG. 5, a gamma correction function in a timing controller disposed in the stage before a source driver, not shown, and incorporate into the source driver a digital-to-analog (D/A) converter that generates an analog voltage in proportion to a logic value received from the timing controller. The D/A converter in the source driver does not have a gamma correction function. Thus, in the source driver, a logic value on which a gamma correction was performed by the timing controller is converted into an analog voltage, which is in turn applied between electrodes in a liquid crystal display.
In the above-described technique, the source driver does not need to have a gamma correction function but may be provided with only the D/A converter that generates an analog voltage proportional to a logic value. Because the timing controller performs a gamma correction, it is not necessary to make a source driver for each liquid crystal display.
Note that the gamma correction in the timing controller generally employs a writable memory such as a random access memory (RAM). The timing controller shown in FIG. 5 uses a gamma correction memory to correct an analog voltage suitable for the characteristics of a liquid crystal display so that it becomes a logic value which is generated in the source driver. The gamma correction data to be used in the correction is written into a writable memory from an external memory such as a read-only memory (ROM) when the liquid crystal display is started.
The request of high image quality to liquid crystal displays becomes stronger and stronger. For this reason, as shown in FIG. 6, a timing controller employs three gamma correction memories for the three primary colors, red, green, and blue. An increase in the number of gamma correction memories, however, enlarges a space that they occupy in the timing controller, which results in an increase in cost.
A technique for solving the above-described problem has been proposed in U.S. patent application publication No. US 2006/0215047 A1 to Miyasaka, which discloses a gamma corrector that receives a digital signal of n bits and outputs a signal of m bits. The gamma corrector includes a first, a second, and a third lookup table, a data coupler and an adder, and has a relationship in which input bits to each lookup table are fewer than n and output bits from each lookup table are fewer than m. More specifically, when signals of x bits and m1 bits are input to and output from the first lookup table, signals of n−1 bits and m2 bits are input to and output from the second lookup table, and signals of n−t bits and k bits are input to and output from the third lookup table, there is a relationship of m≦m1+m2, x<n−t, and m≧m1+k. The data coupler outputs a bit sequence in which a bit sequence from the first lookup table is arranged on the more significant bit side, a bit sequence from the third lookup table is arranged on the less significant bit side, and (m−m1−k) bits of “zero” are interposed between the more and less significant bit sides. The adder adds an output value from the second lookup table and a bit sequence output from the data coupler together and then outputs the added data.
However, in the technique taught by Miyasaka, divided lookup tables are employed for expressing the respective color components. Because of this, it is necessary to divide input image data and then input them to the lookup tables. After gamma correction, the final gamma-corrected image data has to be obtained by a complex method. In gamma correctors, this complex processing has become an important consideration.